1. Field of the Invention
This invention relates to power transmission belts and, more particularly, to power transmission belts having load carrying fiber cords therein.
2. Background Art
Aramid cords are being widely used as load carrying elements in power transmission belts. Aramid fiber has higher strength and a higher modulus than many other organic fibers. Aramid fiber also has excellent dimensional stability. One drawback with aramid cords being used in power transmission belts is that the cords are prone to fraying. This is particularly a problem in belts which have cut, pulley-engaging side surfaces so that the belt rubber, and the load carrying cords embedded therein, are directly exposed to cooperating pulleys in operation. This construction is typical of all types of power transmission belts, including V-belts, V-ribbed belts, and toothed belts. In this environment, the cords may also break loose from the side surfaces of the belts. Many studies have been undertaken to address this problem. Many of these studies have been specifically focused on the automotive industry, in which there is a demand for load carrying cords exhibiting good adhesion, minimal fraying, and good bending fatigue resistance.
To enhance the performance of the load carrying cords, it is known to treat the cords using resorcinol-formalin-rubber latex adhesive liquid (hereinafter RFL liquid). It is also known to pre-treat the load carrying cords with an epoxy compound, an isocyanate compound, or the like, before subjecting the cords to the RFL liquid treatment.
In Japanese Patent Publication No. H.7-72578, a cord is disclosed having initially untwisted ribbon-shaped 300-3100 denier aramid fiber filaments that are adhesive treated with a liquid that is either an epoxy or isocyanate compound. The filaments are lower twisted into strands, with two of the strands then upper twisted. The twisted strands are then either adhesive treated with rubber cement or treated with an RFL liquid followed by an adhesive treatment with rubber cement The coefficient (X) of the upper twist is 1-4 with the coefficient (Y) of the lower twist being -1-1. The upper twist coefficient (X) and the lower twist coefficient (Y) are set in a specific relationship to improve cohesion and improve the fraying problem.
It has been found that in using an RFL liquid treatment alone, the bending performance of the cords is good, but they tend to fray. It has also been found that using RFL liquid after pre-treating with an epoxy isocyanate compound tends to harden the cord. While this improves the fraying properties, the bending fatigue resistance is unsatisfactorily diminished.
Further, by controlling the twists of the cord components, the fraying properties have been improved, however, this alone does not make the aramid fibers suitable as load carrying members in a power transmission belt in the automotive environment.
After much study by the applicant of mechanical properties of a conventionally treated cord, it has been found that there are many voids in these cords. It has been determined that these voids cause fraying to occur.